During the past decade a growing body of evidence has accumulated to indicate that glial cells, and in particular astrocytes, play active roles in information processing (Haydon, 2001). Since astrocytes are juxtaposed with the capillaries and with end-feet on endothelia, and since astrocytic processes enwrap synaptic terminals, it is likely that astrocytes serve regulatory functions in controlling blood flow and synaptic transmission (Haydon, 2001; Raichle, 2001). We hypothesize that astrocytic calcium levels are the key integrative signal for the regulation of these two diverse functions. Specifically, we hypothesize that neuronal activity-induced astrocytic calcium signaling regulates: 1) the synthesis within the astrocyte of the vasodilator nitric oxide (NO), and 2) a feedback regulation of the synapse mediated by the calcium-dependent release of glutamate from astrocytes. Using calcium and nitric oxide imaging, confocal microscopy, electrophysiology, photolysis and adenovirus to overexpress SNARE protein fragments and G-protein-coupled receptors we will test four hypotheses: 1: Physiological calcium signaling in astrocytes stimulates nitric oxide production, which in turn regulates calcium homeostasis. 2: Neuronal activity causes the synthesis of nitric oxide in astrocytes. 3: SNARE proteins are essential for the release of glutamate from astrocytes. 4: The release of glutamate from astrocytes modulates synaptic transmission in hippocampal slices. By performing these studies we will obtain new insights into the roles of astrocytes in the CNS. Since astrocytes can integrate neuronal inputs and release glutamate in response to elevated internal calcium, the demonstration of a role for astrocytes in the control of the synapse will change the way we view information processing in the nervous system.